Zig Guide

Zig Guide

Applies to: Zig 0.13+, Systems Programming, CLIs, Embedded, Game Engines, C/C++ Replacement

Core Principles

1. Explicit Over Implicit: No hidden control flow, no hidden allocations, no operator overloading, no implicit conversions
2. Allocator Passing: Every function that allocates receives an std.mem.Allocator as a parameter; never use a global allocator
3. Errors Are Values: Use error unions (!) with try, catch, and errdefer; never silently discard errors
4. Comptime Over Runtime: Move computation to compile time with comptime; use it for generics, validation, and code generation
5. Safety With Escape Hatches: Keep runtime safety enabled by default; disable only in measured hot paths with a justifying comment

Guardrails

Code Style

• Run zig fmt before every commit (non-negotiable)
• camelCase functions/variables, PascalCase types/structs/enums, SCREAMING_SNAKE_CASE module constants
• Prefer snake_case for file names (my_module.zig)
• Discard unused values explicitly: _ = value; (no _ prefix naming)
• Prefer const over var everywhere; mutability requires justification
• All public declarations (pub fn, pub const) must have a doc comment (///)

Memory and Allocators

• Every function that heap-allocates must accept an std.mem.Allocator parameter
• Always pair alloc/free with defer or errdefer at the call site
• Use std.heap.ArenaAllocator for batch allocations freed together
• Use std.heap.GeneralPurposeAllocator in debug builds (detects leaks, double-free)
• Never store an allocator in a struct unless the struct owns long-lived memory
• Prefer stack allocation (var buf: [256]u8 = undefined;) for small, bounded buffers
• Slices ([]T, []const T) over raw pointers ([*]T) whenever possible

Error Handling

• Return error unions (!T) for all fallible operations
• Use try to propagate; use catch only where you can handle or log meaningfully
• Use errdefer to clean up resources on error paths
• Define domain-specific error sets; avoid anyerror except at top-level boundaries
• Never ignore errors: _ = fallibleFn(); is forbidden outside tests

const ConfigError = error{ FileNotFound, InvalidFormat, MissingField };

fn loadConfig(allocator: std.mem.Allocator, path: []const u8) ConfigError!Config {
    const file = std.fs.cwd().openFile(path, .{}) catch return error.FileNotFound;
    defer file.close();
    // parse and return ...
}

Comptime

• Use comptime parameters for generics instead of runtime polymorphism
• Use @typeInfo and @Type for type introspection at compile time
• Use @compileError to produce clear messages for invalid type combinations
• Prefer inline for only when the loop count is comptime-known and small

fn max(comptime T: type, a: T, b: T) T {
    return if (a > b) a else b;
}

fn ensureUnsigned(comptime T: type) void {
    if (@typeInfo(T) != .int or @typeInfo(T).int.signedness == .signed)
        @compileError("expected unsigned integer type");
}

Safety

• Keep runtime safety enabled in debug and test builds
• Validate all external inputs (file I/O, network, FFI boundaries)
• Use std.math.add / std.math.mul for checked arithmetic; sentinel slices ([:0]const u8) for C strings
• Avoid @ptrCast and @intFromPtr unless interfacing with C; justify each use

Key Patterns

Optionals

fn findUser(users: []const User, id: u64) ?*const User {
    for (users) |*user| {
        if (user.id == id) return user;
    }
    return null;
}

const user = findUser(users, 42) orelse return error.UserNotFound;

if (findUser(users, 42)) |u| {
    std.debug.print("Found: {s}\n", .{u.name});
}

Defer and Errdefer

fn createConnection(allocator: std.mem.Allocator, host: []const u8) !*Connection {
    const conn = try allocator.create(Connection);
    errdefer allocator.destroy(conn); // only runs on error return

    conn.* = .{ .socket = try std.net.tcpConnectToHost(allocator, host, 8080), .allocator = allocator };
    errdefer conn.socket.close();

    try conn.performHandshake();
    return conn;
}

Slices vs Arrays

const fixed: [4]u8 = .{ 1, 2, 3, 4 };          // fixed-size array (stack)
const c_str: [:0]const u8 = "hello";             // sentinel-terminated (C compat)

fn sum(values: []const u32) u64 {                // slice (preferred for params)
    var total: u64 = 0;
    for (values) |v| total += v;
    return total;
}

Packed and Extern Structs

const StatusRegister = packed struct {
    carry: u1, zero: u1, irq_disable: u1, decimal: u1,
    brk: u1, _reserved: u1, overflow: u1, negative: u1,
};
const CFileHeader = extern struct { magic: u32, version: u16, flags: u16, size: u64 };

Testing

const testing = @import("std").testing;

test "add returns correct sum" {
    try testing.expectEqual(@as(i32, 5), add(2, 3));
}

test "string list detects leaks via testing allocator" {
    var list = StringList.init(testing.allocator); // leak detection built-in
    defer list.deinit();
    try list.add("hello");
    try testing.expectEqual(@as(usize, 1), list.items.items.len);
}

test "readFileContents returns error for missing file" {
    try testing.expectError(error.FileNotFound, readFileContents(testing.allocator, "/nonexistent"));
}

Testing Standards

• Test names describe behavior: test "user creation fails with empty name"
• Unit tests live in test blocks inside the same .zig file
• Integration tests go in tests/ directory
• Use std.testing.allocator -- it detects memory leaks automatically
• Key assertions: expectEqual, expectError, expectEqualStrings, expectEqualSlices
• Coverage target: >80% for library code, >60% for application code

Tooling

zig build              # Build (uses build.zig)
zig build test         # Run all tests
zig build run          # Build and run
zig fmt src/           # Format all source files
zig test src/main.zig  # Test a single file
zig build -Doptimize=ReleaseSafe   # Optimized with safety
zig build -Doptimize=ReleaseFast   # Maximum performance

See patterns.md for a build.zig template.

C Interop

• Use @cImport / @cInclude for C headers (not manual extern declarations)
• Use [*c]T only at FFI boundary; convert to []T or ?*T immediately
• Use std.mem.span to convert [*:0]const u8 (C string) to Zig slice
• Always call exe.linkLibC() in build.zig when using libc
• Validate all values received from C before using in safe Zig code

References

For full implementation examples, see:

• references/patterns.md -- Arena allocator, GPA leak detection, comptime generics, comptime interface validation, C library wrapper (SQLite), sentinel string conversion, tagged union state machines, errdefer chains, build.zig template

External References

• Zig Language Reference
• Zig Standard Library Docs
• Zig Learn
• Zig by Example
• Zig Cookbook